Evolution and motions of magnetic fragments during the active region formation and decay: A statistical study

¹ Charles University, Astronomical Institute, V Holešovičkách 2, 18000 Prague 8, Czech Republic
e-mail: svanda@sirrah.troja.mff.cuni.cz
² Astronomical Institute of the Czech Academy of Sciences, Fričova 298, 25165 Ondřejov, Czech Republic
³ Charles University, Department of Surface and Plasma Science, V Holešovičkách 2, 18000 Prague 8, Czech Republic

Received: 14 December 2020
Accepted: 1 February 2021

Abstract

Context. The evolution of solar active regions is still not fully understood. The growth and decay of active regions have mostly been studied in case-by-case studies.

Aims. Instead of studying the evolution of active regions case by case, we performed a large-scale statistical study to find indications for the statistically most frequent scenario.

Methods. We studied a large sample of active regions recorded by the Helioseismic and Magnetic Imager instrument. The sample was split into two groups: forming (367 members) and decaying (679 members) active regions. We tracked individual dark features (i.e. those that are assumed to be intensity counterparts of magnetised fragments from small objects to proper sunspots) and followed their evolution. We investigated the statistically most often locations of fragment merging and splitting as well as their properties.

Results. Our results confirm that statistically, sunspots form by merging events of smaller fragments. The coalescence process is driven by turbulent diffusion in a process similar to random-walk, where supergranular flows seem to play an important role. The number of appearing fragments does not seem to significantly correlate with the number of sunspots formed. The formation seems to be consistent with the magnetic field accumulation. Statistically, the merging occurs most often between a large and a much smaller object. The decay of the active region seems to take place preferably by a process similar to the erosion.